Friction clutches are frequently constructed of interdigited or interleaved friction plate elements which are compressed towards each other. One group of the elements is secured to a shaft, the other group of elements is secured to an outer ring, coaxial with the shaft. When the ring elements and the shaft elements are pressed together, a friction connection of the clutch is effected.
Engagement and disengagement is controlled by a positioning element which can be deflected between a plane, essentially flat, radially extending position to a conical position. This arrangement permits application of force on the positioning plate along, for example a central portion thereof, with lever multiplication of the force in radially spaced regions. Such plates, typically ring plates, surrounding one of the shafts of the clutch are described in the referenced U.S. Pat. No. 2,725,964, to which German Patent 838,094 corresponds.
Ring plates or ring disks which permit deformation between a flat, planar shape, in accordance with the prior art, are formed with slits, which alternately extend from an inner or central opening towards the circumference, and from the circumference towards the central opening. These slits are usually longer than half of the radial width of the ring disk. The result of these slits will be the formation of radially extending lever elements, which can operate similarly to double-arm levers or knee levers. Thus, a comparatively small external force, applied in the central region by axial shifting of an operating element, can obtain high compressive force at a circumferential region which can then be applied to a plurality of friction clutch disks. The referenced U.S. Pat. No. 2,725,964 shows such an overall structure and having the foregoing advantage.
Modern high-power machinery requires transfer of higher and higher torques through such clutches and, thus, also require higher and higher surface pressure forces. These forces, preferably, should be capable of being transferred by clutches which have an outer diameter no larger than clutches developed for transfer of lower forces.
To transfer higher torques, the number of friction surfaces brought into engagement with each other can be increased. As the number of friction plates increases, the compression path will have a longer clutching zone between disengaged and engaged positions. For disengaged positions in multi-layer clutch structures, a certain predetermined minimum distance between the pairs of the friction elements is required.
There are limits beyond which changes in material thickness will not result in proportionally increased transfer of forces. If a ring disk is made thicker, the torque transfer capability does not rise linearly with the increase in thickness thereof. Apparently, the radial elements formed by the alternate placement of the radial slits in the ring disks are subject to a bending load caused by the actuating force of the clutch. However, each radial element is connected, at its radially inner and outer end, to a neighboring radial element. When the ring disk switches in position from a plane to a cone, a torsional loading of the elements occurs. Superimposed on this torsional load and on the bending load mentioned above is a load extending in circumferential direction of the ring disk. This circumferential load arises since, upon switch-over between disengaged and engaged position, the size of the inner circumference decreases as the disk is changed from planar to conical shape or position. Thus, an additional bending load is applied to the radially extending disk elements, projecting between the respective slits, and placing a load in circumferential direction.
Increasing the thickness of such a disk, and thus strengthening the radial disk elements, thus does not lead to a proportional increase in torque transferring capability. Of course, the thickness of the disk cannot be increased beyond some predetermined values.
Providing more slits and narrower disk elements likewise will not lead to a proportional increase in clutching force. The radial slits of the ring disks, in order to prevent deformation and notch effects, and stresses at end of the slits which may lead to fissures, must be terminated with comparatively large-diameter essentially circular surface contours. The size of these surface contours, again, depends on the thickness of the ring disk. As the thickness of the ring disks increases, the contours must be formed with diameters which likewise increase so that fissures, which might arise due to notch effects and localized stresses can be avoided. Consequently, the spacing between adjacent radially extending elements increases or, alternatively, the danger of fissures due to notch effects increases.
Comparatively large-diameter end surfaces require a comparatively wide slit which, again, reduces the width of the elements of the ring disks which in turn reduces the positioning surface available in the outer circumferential regions. Reducing these positioning surfaces results in a reduction of force transfer capability, so that increase of torque or rotary force cannot be obtained by merely increasing the thickness of the material of the ring disk beyond design values which result from compromise between material damage and material fatigue at the ends of the slits and width of the slits.
The above discussion is applicable to all ring disks of the known prior art type. The width of the radial slits, in the end, must be determined by the required dimensions of the ends of the slits to avoid the danger of fissures and notch effect and localized stresses, so that increase of the positioning surface by decreasing the width of the slit is limited. Additionally, the radially projecting elements of the ring disks are stressed both in torsion as well as in bending, which combined stresses limit the power or force transfer capability. Consequently, increasing the torque transfer capability of clutches using such ring disks by merely changing its material and/or size and thickness has economic and technical limits.